DESCRIPTION: In the two-stage model of cutaneous carcinogenesis, a single subtumorigenic exposure to a carcinogen (initiation) and subsequent chronic regenerative epidermal hyperplasia of sufficient magnitude (promotion) can induce benign and malignant neoplasms. Tumor initiation is thought to convert some of the keratinocytes into latent neoplastic cells; promotion elicits expression of the neoplastic change. The identity of the latent neoplastic cel has long been a mystery. The hypothesis to be tested is that follicular kerati 19 (Kl9) -expressing keratinocytes are the cutaneous multipotential progenitor and are the target cells in two-stage carcinogenesis. The Investigators' objective is to test this hypothesis and to determine whether these cells have other characteristics of stem cells. To this end, they have developed two nove and useful approaches. The specific aims are as follows. 1. Identify, isolate, and characterize the stem cell properties of the K19-expressing cells by using the Kl9 promoter sequence to express green fluorescent protein in transgenic mice. 2. Determine whether Kl9-expressing cells are the multipotential progenitors of the cutaneous epithelium by employing a FLP recombinase lineage mapping strategy known to be effective in cultured cells and in transgenic mice. Empower this strategy by temporally regulating the expression of FLP, thereby enabling the lineage analysis of cellular replacement in the cutaneous epithelium of adult transgenic mice. 3. Determine whether the Kl9 expressing cells are the latent neoplastic cells in carcinogen-exposed skin by using the temporally regulated lineage mapping strategy of Specific Aim 2 to induce FLP transiently in the Kl9 expressing cells at the time of tumor initiation, thus tracking the development of neoplasms from the presumptive progenitors. Identification and isolation of the cutaneous multipotential progenitors opens the door to health-related research of great significance including: regulatio of keratin and cell adhesion genes in terminal differentiation, gene therapy, the role of the stem cells in development, hair growth and wound healing, repair of carcinogen-DNA adducts, and manipulation of stem cell self-renewal. Furthermore, their novel strategy for inducible lineage mapping has applications beyond the Kl9 promoter or even other keratin promoters. The method will work with any promoter expressed by any progenitor cell in any organ system, thus enabling the exploration of a vast number of lineage relationships in developmental biology and in cancer research.